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Leonid Komarianskyi
4b8a11b421
When the start address is not cacheline aligned, the bytes covered by
the first, partial cacheline are subtracted from 'size'. Nothing checks
that the region actually extends beyond that cacheline, so if it does
not, the unsigned subtraction underflows and the loop that follows walks
practically the entire address space.
For example, with a 64-byte cacheline, p = 0x40000001 and size = 1:
p & cacheline_mask -> 1, so the partial head is taken
size -= 64 - 1 -> 1 - 63 underflows to 0xffffffffffffffc2
Instead of a single "dc civac" at 0x40000000, the loop then runs
288230376151711743 times, issuing "dc ivac" from 0x40000040 upwards.
Invalidating without cleaning discards dirty cachelines, so this
corrupts whatever is mapped along the way until Xen takes a data abort.
Moreover, _cache_flush() in common/grant_table.c, reachable from a guest
via GNTTABOP_cache_flush, bounds 'offset' and 'length' against the page
size but requires no alignment, so GNTTAB_CACHE_INVAL with offset = 1
and length = 1 is enough to trigger this. Note that the page may belong
to a different domain (the d != owner path), so the data being discarded
is not necessarily the caller's own.
The subtraction dates back to the fix for XSA-437, so this is a regression
in that change.
Fix this by detecting the underflow instead of letting it happen: if the
region ends inside the first cacheline, there is nothing left to flush
and the function bails out right after cleaning it. Partially covered
cachelines are still cleaned as well as invalidated, so that data outside
of the region is not discarded.
Additionally, drop 'idx' and advance 'p' directly instead. It simplifies
the logic while using fewer variables and less arithmetic.
Note that the partial head is now cleaned using the unaligned start of the
region instead of the start of its cacheline. This is permitted by the
architecture; the Arm ARM (DDI 0487) states:
"When a DC instruction requires an address argument this takes the form
of a 64-bit register that holds the VA argument. No alignment
restrictions apply for this address."
The same applies on Arm32.
Also, switch the asm() in the loop to asm_inline, like the neighbouring
ones: the wrapper expands to a single instruction with a trailing
semicolon, which GCC counts as two statements, overestimating the size
of the function when deciding whether to inline it.
This is CVE-2026-102699, part of XSA-519.
Fixes: 9a216e92de ("xen/arm: page: Handle cache flush of an element at the top of the address space")
Signed-off-by: Leonid Komarianskyi <leonid_komarianskyi@epam.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Julien Grall <julien@xen.org>
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https://www.xen.org/
What is Xen?
============
Xen is a Virtual Machine Monitor (VMM) originally developed by the
Systems Research Group of the University of Cambridge Computer
Laboratory, as part of the UK-EPSRC funded XenoServers project. Xen
is freely-distributable Open Source software, released under the GNU
GPL. Since its initial public release, Xen has grown a large
development community, spearheaded by xen.org (https://www.xen.org).
This file contains some quick-start instructions to install Xen on
your system. For more information see https://www.xen.org/ and
https://wiki.xen.org/
Quick-Start Guide
=================
First, this is just a quick-start guide. For more comprehensive
information see the INSTALL file and the Xen wiki at
https://wiki.xenproject.org and in particular
https://wiki.xenproject.org/wiki/Getting_Started.
Second, there are a number of prerequisites for building a Xen source
release. Make sure you have all the following installed, either by
visiting the project webpage or installing a pre-built package
provided by your OS distributor:
* GNU Make v3.80 or later
* C compiler and linker:
- For x86:
- GCC 5.1 or later
- GNU Binutils 2.25 or later
or
- Clang/LLVM 11 or later
- For ARM:
- GCC 5.1 or later
- GNU Binutils 2.25 or later
- For RISC-V 64-bit:
- GCC 12.2 or later
- GNU Binutils 2.39 or later
* POSIX compatible awk
* Development install of zlib (e.g., zlib-dev)
* Development install of Python 2.7 or later (e.g., python-dev)
* Development install of curses (e.g., libncurses-dev)
* Development install of uuid (e.g. uuid-dev)
* Development install of json-c 0.15 or later (e.g. libjson-c-dev)
or yajl (e.g. libyajl-dev)
* Development install of libaio (e.g. libaio-dev) version 0.3.107 or
greater.
* Development install of GLib v2.0 (e.g. libglib2.0-dev)
* Development install of Pixman (e.g. libpixman-1-dev)
* pkg-config
* bridge-utils package (/sbin/brctl)
* iproute package (/sbin/ip)
* GNU bison and GNU flex
* ACPI ASL compiler (iasl)
In addition to the above there are a number of optional build
prerequisites. Omitting these will cause the related features to be
disabled at compile time:
* Binary-search capable grep (if building Xen with CET support)
* Development install of Ocaml (e.g. ocaml-nox and
ocaml-findlib). Required to build ocaml components which
includes the alternative ocaml xenstored.
* cmake (if building vtpm stub domains)
* pandoc, transfig, pod2{man,html,text} for rendering various pieces of
documentation into alternative formats
* figlet (for generating the traditional Xen start of day banner)
* systemd daemon development files
* Development install of libnl3 (e.g., libnl-3-200,
libnl-3-dev, etc). Required if network buffering is desired
when using Remus with libxl. See docs/README.remus for detailed
information.
* 16-bit x86 assembler, loader and compiler for rombios
(dev86 rpm or bin86 & bcc debs)
* Development install of liblzma for rombios
* Development install of libbz2, liblzma, liblzo2, and libzstd for DomU
kernel decompression.
Second, you need to acquire a suitable kernel for use in domain 0. If
possible you should use a kernel provided by your OS distributor. If
no suitable kernel is available from your OS distributor then refer to
https://wiki.xen.org/wiki/XenDom0Kernels for suggestions for
suitable kernels to use.
If you are looking to compile a Dom0 kernel from source, please refer to
https://wiki.xen.org/wiki/XenParavirtOps.
[NB. Unless noted otherwise, all the following steps should be
performed with root privileges.]
1. Download and untar the source tarball file. This will be a
file named xen-unstable-src.tgz, or xen-$version-src.tgz.
You can also pull the current version from the git or mercurial
repositories at https://xenbits.xen.org/
# tar xzf xen-unstable-src.tgz
Assuming you are using the unstable tree, this will
untar into xen-unstable. The rest of the instructions
use the unstable tree as an example, substitute the
version for unstable.
2. cd to xen-unstable (or whatever you sensibly rename it to).
3. For the very first build, or if you want to destroy build trees,
perform the following steps:
# ./configure
# make world
# make install
See the documentation in the INSTALL file for more info.
This will create and install onto the local machine. It will build
the xen binary (xen.gz), the tools and the documentation.
You can override the destination for make install by setting DESTDIR
to some value.
4. To rebuild an existing tree without modifying the config:
# make dist
This will build and install xen, tools, and docs into the local dist/
directory.
You can override the destination for make install by setting DISTDIR
to some value.
make install and make dist differ in that make install does the
right things for your local machine (installing the appropriate
version of udev scripts, for example), but make dist includes all
versions of those scripts, so that you can copy the dist directory
to another machine and install from that distribution.
xenstore: xenstored and oxenstored
====================================
Xen uses a configuration database called xenstore [0] to maintain configuration
and status information shared between domains. A daemon is implemented as part
of xenstore to act as an interface for access to the database for dom0 and
guests. Two xenstored daemons are supported, one written in C which we refer
to as the xenstored (sometimes referred to as cxenstored), and another written
in Ocaml called oxenstored. Details for xenstore and the different
implementations can be found on the wiki's xenstore reference guide [1] and
the xenstored [2] page. You can choose which xenstore you want to enable as
default on a system through configure:
./configure --with-xenstored=xenstored
./configure --with-xenstored=oxenstored
By default oxenstored will be used if the ocaml development tools are found.
If you enable oxenstored the xenstored will still be built and installed,
the xenstored used can be changed through the configuration file:
/etc/sysconfig/xencommons
or
/etc/default/xencommons
You can change the preferred xenstored you want to use in the configuration
but since we cannot stop the daemon a reboot will be required to make the
change take effect.
[0] https://wiki.xen.org/wiki/XenStore
[1] https://wiki.xen.org/wiki/XenStoreReference
[2] https://wiki.xen.org/wiki/Xenstored
Python Runtime Libraries
========================
Various tools, such as pygrub, have the following runtime dependencies:
* Python 2.7 or later.
URL: https://www.python.org/
Debian: python
Intel(R) Trusted Execution Technology Support
=============================================
Intel's technology for safer computing, Intel(R) Trusted Execution Technology
(Intel(R) TXT), defines platform-level enhancements that provide the building
blocks for creating trusted platforms. For more information, see
https://www.intel.com/technology/security/.
Intel(R) TXT support is provided by the Trusted Boot (tboot) module in
conjunction with minimal logic in the Xen hypervisor.
Tboot is an open source, pre- kernel/VMM module that uses Intel(R) TXT to
perform a measured and verified launch of an OS kernel/VMM.
The Trusted Boot module is available from
http://sourceforge.net/projects/tboot. This project hosts the code in a
mercurial repo at http://tboot.sourceforge.net/hg/tboot.hg and contains
tarballs of the source. Instructions in the tboot README describe how
to modify grub.conf to use tboot to launch Xen.